It is known that a nanometer-size particulate material can exhibit properties distinct from those of the corresponding bulk material. For example, a semiconductor is well known for the so-called quantum size effect, in which the band gap, which had been believed to be material-specific, varies with particle size. The particle size at which this effect is significant is generally from a few nm to tens of nm, depending on the type of semiconductor material. Thus, a singlenanoparticle is particularly important. Some materials are also known for another effect, in which, as the quantum size effect becomes significant, the fluorescence lifetime becomes short, and a certain luminescence becomes to be observed, which would otherwise not be observed. As mentioned above, nanosized materials, in particular single-nanosized materials, can exhibit properties different from the known properties of the corresponding bulk materials, and thus are attracting widespread attention in science and engineering.
A certain semiconductor nanoparticle phosphor material is proposed that comprises a semiconductor nanoparticle, for example, of CdSe/CdS (core/shell), CdSe/ZnS (core/shell), or the like, and the semiconductor nanoparticle is utilized to form beads whose surface is coupled to a molecule probe for detecting a target molecule (see, for example, Science, Vol. 281, No. 25, 1998, pp. 2013-2016, and Nature Biotechnology, Vol. 19, 2001, pp. 631-6354). These semiconductor nanoparticles of different crystallite sizes can produce different wavelengths of emission. If the labeled beads are encoded with respect to a combination of luminescence wavelength and luminescence intensity, simultaneous multiple measurements are possible. The semiconductor nanoparticle phosphor material has good properties for a labeling material, such as high sensitivity, low cost, and easiness of automation. Using the semiconductor nanoparticle phosphor material as a labeling material, therefore, a specific site of a living organism, a certain substance in plasma, or the like, can be detected at high sensitivity and high speed.
Another semiconductor nanoparticle phosphor material is proposed that comprises a semiconductor nanoparticle whose surface is coated with a modifying molecule, so as to have improved affinity to the matrix (for example, see U.S. Pat. No. 6,319,426, JP-A-2002-38145 (“JP-A” means unexamined published Japanese patent application), JP-A-2003-64278; and Science, Vol. 281, No. 25, 1998, pp. 2016-2018). The semiconductor nanoparticle coated with the modifying molecule can have improved affinity to an aqueous medium, and/or improved dispersibility in an organic macromolecule or an organic solvent. Thus, the semiconductor nanoparticle phosphor material can easily be applied as a labeling material, and a luminescent material can easily be produced by dispersing the semiconductor nanoparticle phosphor material into a resin. The semiconductor nanoparticle phosphor material is therefore expected to be widely applied in the fields of optical devices, clinical diagnosis, biochemical research or medical science research, or the like.
However, the use of the semiconductor nanoparticle phosphor material of CdSe or CdSe/ZnS (core/shell) or the like may raise safety issues and environmental issues. Therefore, alternatives are desired that are safe and have less influence on the environment. A useful alternative is a nanoparticle phosphor material of zinc sulfide (ZnS) doped with manganese ion (Mn2+) or the like, which material can easily be synthesized in a solvent, such as water. Compared with the case of the above semiconductor nanoparticle phosphor material, it is difficult to control the luminescence wavelength of the zinc sulfide (ZnS) nanoparticle phosphor material by varying its crystallite size. On the other hand, the ZnS nanoparticle phosphor material is advantageous in that its luminescence wavelength can be varied with the type of the doping metal ion or the surface-modifying molecule (surface modifier) (for example, see JP-A-2002-322468; Journal of the Illuminating Engineering Institute of Japan, Vol. 87, No. 4, 2003, pp. 256-261; and Journal of The Electrochemical Society, Vol. 149, No. 3, 2002, pp. H72-H75).
However, the zinc sulfide (ZnS)-based nanoparticle phosphor material has a relatively large surface area, such that it can significantly cause secondary aggregation, thus the zinc sulfide (ZnS)-based nanoparticle phosphor material cannot easily form a transparent colloidal dispersion, and it can hardly be functionalized for the purpose of application to fluorescent labeling materials or luminescent devices. JP-A-2002-38145 discloses ZnS-based nanoparticles having a specific amino group-containing compound fixed on the surface by condensation reaction. However, it has been found that such a compound does not always have sufficient dispersibility in a specific solvent, such as water.